Chapter 17: Invertebrate Animals

The segment passes with wastes out of the host’s body. If
another host eats a fertilized egg, it hatches and develops into a
tapeworm. Tapeworm segments aren’t ingested directly by
humans. Most flatworms have an intermediate or middle host.
For example, Figure 8 shows how cattle are the intermediate
host for tapeworms that infect humans.
How can flatworms get into humans
Roundworms
If you have a dog, you may know already that heartworm
disease, shown in Figure 9, can be fatal to dogs. In most areas of
the United States, it’s necessary to give dogs a monthly medicine
to prevent heartworm disease. Heartworms are just one kind of
the many thousands of roundworms that exist. Roundworms
are the most widespread animal on Earth. Billions can live in an
acre of soil. Many people confuse earthworms and roundworms.
You will study earthworms in the next section.
A roundworm’s body is described as a tube within a tube,
with a fluid-filled cavity in between the two tubes. The cavity
separates the digestive tract from the body wall. Roundworms
are more complex than flatworms because their digestive tract
has two openings. Food enters through the mouth, is digested in
a digestive tract, and wastes exit through the anus.
Roundworms are a diverse group. Some roundworms are
decomposers, others are predators, and some, like the heartworm,
are animal parasites. Other roundworms are plant parasites.
Figure 9 This dog heart is
infested with heartworms.
Heartworms are carried by
mosquitoes. A heartworm
infection can clog a dog’s
heart and cause death.
Summary
Sponges
•
and Cnidarians
Sponges are animals that remain attached to
one place and can reproduce both sexually
•
and asexually.
Cnidarians are hollow-bodied animals with
two cell layers that are organized into tissues.
•
Flatworms and Roundworms
Flatworms have three layers of soft tissue
organized into organs and organ systems.
•
Most flatworms are parasitic.
Roundworms are decomposers, predators, or
parasites of plants and animals and are the
most widespread animal on Earth.
Self Check
1. Explain how sponges and cnidarians get food.
2. Compare and contrast the body plan of flatworms to
the body plan of roundworms.
3. Infer how spongin and spicules discourage predators
from eating sponges.
4. Think Critically Some types of sponges and cnidarians
reproduce asexually. Why is this beneficial to them
5. Solve an Equation A sponge is 1 cm in diameter
and 10 cm tall. It can move 22.5 L of water through its
body in a day. Calculate the volume of water it pumps
through its body in 1 min.
red.msscience.com/self_check_quiz
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Renee Stockdale/Animals Animals